Anyone familiar with the drilling arts is well acquainted with the "tri-cone" drill bit. The "tri-cone" bits are so named because the cutting elements consist of three cones, studded about their conical surface with teeth, or for harder rock, with tungsten carbide buttons. The genesis of such bits was the dual cone bit developed by Howard Hughes, Sr., and introduced in 1909. That dual cone type bit had sharp concentric rings about the cone. Later, in the 1930's, a third cone was added, and the design became a "tri-cone". When sintered tungsten carbide became available, such cones were fitted with protruding carbide buttons in a variety of shapes and patterns. Such conical cutters are sometimes referred to as "raspberry cutters" because their appearance is vaguely suggestive of raspberries. Over the years, various improvements have been made in bearings, seals, lubricants, and in the tungsten carbide alloys and shapes. Still, however, the basic "tri-cone" bit is the primary bit design used in drill bits for drilling through hard rock today.
In the early 1950's, tunnel boring machines ("TBMs") attempted to attack harder rock formations. To do so, many TBMs were equipped with multi-row rings of steel tooth or carbide button cutters; such cutters were initially based on drill bit cutting tool experience. However, in 1956, on a sewer drilling project in Toronto, Canada, a TBM unit was equipped with single disc cutters, and in using such cutters, set an impressive record of one hundred five (105) feet [32 meters] distance bored through rock in one day. Resultingly, by 1979, the remaining TBM manufacturers equipped their machines with single rolling disc cutters.
A similar situation occurred in large diameter rotary drilling, such as is used for excavating a mine shaft. In 1979, a ninety nine (99) inch [251.5 cm] drill head equipped with all rolling disc cutters was successfully demonstrated, and it set advance rate records in hard limestone. As in the tunnel boring industry, that technology is now employed by virtually all commercial big hole drilling operators.
The fundamental rationale for the productivity of the single cutting edge rolling disc cutter technology can be understood by reference to FIG. 1. The graph provided in FIG. 1 shows the relationship between energy required for drilling as a function of the mean particle size of the chip or cuttings created by the excavation tool. Significantly, when the average chip size is large, the energy required to excavate a give amount of rock is small. Conversely, if the tool grinds the rock into very small particles of sand or powder, the specific energy of excavation is high. Another way to look at the situation is that if the cutting machinery consumes considerable power grinding the rock to powder, the rate of advance will be slow. Fundamentally, to improve the rate of advance without increasing the power requirements, larger size cuttings must be created.
In an instrumented test, we have found that a typical "off-the-shelf" tri-cone bit of nine and one-quarter inch (91/4") [23.5 cm] size required a specific energy of eighty (80) horsepower-hour per ton (hp-hr/ton) in well cured concrete. For drilling in basalt, the same tri-cone bit consumed a specific energy of one hundred twenty (120) hp hr/ton. Such bits expend a considerable portion of their power input in the crushing and grinding of the rock being excavated. Since larger diameter cutterheads equipped with rolling disc cutters presently routinely achieve three (3) to seven (7) hp-hr/ton, it can be appreciated that it would be desirable to improve the specific energy of excavation in small diameter rotary drill bits.
Several attempts have been made which to some limited extent tried to provide the desired results, and some of such apparatus superficially resembles the present invention to some small degree. First, early in the history of the Hughes Tool Company, a bit containing two thin disc cutters mounted on a bit body was built and tested. The discs were mounted one on each side of the bit, and gouged the ground in a rolling, scraping motion. However, the discs did not engage the ground in multiple concentric kerfs to form chip type cuttings, but excavated rock by a scouring action. That technique is feasible only for soft materials, and would not long work in rock. Thus, it was never commercialized, evidently because other designs are more satisfactory, even in soft ground. Second, there are some drill bit designs, formerly quite common but now largely phased out of use, which utilize cones with multiple sharpened edges. Those designs have been referred to by some as "disc cutters", and produce concentric circles in the rock face, and do excavate with a chipping action. See the prior art bit cone 25 shown in FIG. 2, for an example. On small diameter cutters, three such multi-row cutters were used, thus conforming to the tri-cone bit arrangement. That design was tried in attempts to form multiple tracks or kerfs in the rock face. However, the design became largely obsolete because it is relatively poor performing compared to the best button type cutters. Now, our novel rolling disc cutter design provides such an improvement over prior art cutters as to make such multi-row cutters totally obsolete.